kieuhieu I see. Well, the passages photographed in the first and the third picture are actually not supposed to be on the beat - they are free ornaments that are supposed to sound light and gracious, not rhythmically accurate. These were commonly expressed in Mertz's time through these sequences of smaller notes that you see on your score.
When it comes to the second picture, the first part of the measure is supposed to be in the same tempo as the music before. Just a steady sixteenth-note beat! The second part is a fast scale, which in this case also functions like a bit of an ornament. Just try to play it as fluidly and graciously as you can, without worrying too much about being on the beat for the 32nd notes.
As for memorization, there are many tricks, but the best one at this stage is to just practice - by the time you've repeated things enough times to sound well, you will have it in your head!
kieuhieu for any live, community, or general tonebase Guitar suggestions, feel free to use my email address, [email protected] . Just be aware though, I'm afraid I won't be able to provide personalized guidance by email at this point. The forums are a better place for all your repertoire and other musical questions!